Add Dense/Sparse annotation to DataType::Union

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
While working on validation for ArrayData (#810), I was trying to validate that the number of Buffers passed to a UnionArray were accurate.

However, since number of Buffers for a UnionArray differs based on if it has a "sparse" or "dense" layout (1 or 2, respectively), the sparseness for the UnionArray is not encoded in the typeDataType::UnionArray; This means we can't easily validate that the UnionArray has the correct number of buffers for its type

It also means that the UnionArray can not be used without first checking to see how many buffers it has (what type it is)

Describe the solution you'd like
I propose changing

enum DataType {
..
  Union(Vec<Field>),
..
}

To something like

enum UnionMode {
  Sparse,
  Dense
}

enum DataType {
..
  Union(UnionMode, Vec<Field>),
..
}

which is both consistent with the C++ implementation as well as allows UnionArray to be statically typechecked: https://github.com/apache/arrow/blob/661c7d749150905a63dd3b52e0a04dac39030d95/cpp/src/arrow/type.h#L1028

Describe alternatives you've considered
Can keep the sparse/dense

Additional context
Add any other context or screenshots about the feature request here.

[alamb]

@nevi-me and @paddyhoran since it looks like you involved in the initial implementation of UnionArray I wonder if you have thoughts.

the arrow2 implementation appears to use a bool to encode sparseness: https://github.com/jorgecarleitao/arrow2/blob/55c3f9cf31f8fd0bc9c49f48d6265f07e2367ec8/src/datatypes/mod.rs#L95-L96

cc @jorgecarleitao

It may be the case that we simply "wait for arrow2" and not do this ticket

[alamb]

Maybe related to #85

[paddyhoran]

Sorry for taking so long. I modeled the original impl on the C++ implementation, it's possible that the C++ impl added this after I wrote the initial implementation. I agree with the change.

It may be the case that we simply "wait for arrow2" and not do this ticket

I've been out of the loop, is there a plan for adopting arrow2?

[alamb]

I've been out of the loop, is there a plan for adopting arrow2?

I would say there are no plans -- lots of hope and ideas though :)